Objective For patients with foot drop gait, the research aims to design a kind of anterior ankle foot orthosis with adjustable stiffness. This kind of orthosis can complete the function of limiting unusual plantar flexion under the condition that the abduction and rotation of ankle joint are limited as little as possible. Methods The minimum orthodontic moment of 10 foot drop male patients was measured by self-made experimental equipment, which can be used to select the optimum material modulus of ankle foot orthosis. The relationship between Young"s modulus and different filling structure and filling ratio parameters was studied by tensile test. A typical patient with foot ptosis was selected, and the anterior orthosis fitting the shape of patient"s foot was quickly made by 3D printing with foot geometric data and specific filling material, filling structure and filling rate. The kinematics and surface electromyography of plantar flexors were tested under barefoot and wearing two kinds of AFOs to verify the effect of the orthosis on plantar flexion. The effectiveness of the limitation and the degree of preservation of ankle valgus and plantar flexion were discussed. Results The minimum corrective torque required for patients with foot drop was 2.16 N.m. Compared with the rigid AFO, the range of motion of plantar flexion and valgus increased by 67.8% and 88.6% respectively when with flexible AFO. The activation of the muscles responsible for plantar flexion (soleus, medial head of gastrocnemius and lateral head of gastrocnemius) also decreased by 38.3%、 46.6% and 55.8%, maintaining a certain normal function. Conclusions This AFO can be used for orthosis customization of patients with foot drop, providing more effective and long-term orthosis function.